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The rate of placement of concrete are always higher, up to hydrostatic pressures, and the concrete more flowable than in the past.

The consistency class plays a crucial role in the maximum pressure definition: The most significant load, especially for a tunnel shutter, is represented by the concrete pressure. This means higher loads acting of the formworks and consequently a bigger importance must be given to its design.

Djn this level, the pressure is considered constant, so an increasing of the depth does not more imply an increasing of the pressure. The shutter must be designed properly in order to ensure that the loads are carried safely with minimum deformations. When the concrete is placed, it exerts a horizontal pressure against the surface of the formwork.

In reality other factors, such as coarse aggregate concentration, aggregate nominal size, concrete temperature, smoothness and permeability of the formwork material, size and shape of the formwork can modify the pressure distribution. The mix design and the cycle of work have to be considered at early stage to ensure safely designed dkn cost-effective formwork systems.

Several studies and methods have been developed to predict the lateral pressure of the concrete. The formwork must withstand several loads such as dead loads self-weight of the structurelive loads, fresh concrete pressure and in some cases wind and snow loads. This is a simplified scheme; it takes into account only three variables: However, all these methods assume that concrete pressure increases linearly with depth to a maximum value and remains constant thereafter.

They are used to cast inner rings of tunnel, walls, slabs, ecc. However, the concrete is fluid only at earl stage of the pour; it starts the bounding process usually when the casting is still underway.

This gives rise to a lower pressure acting on the vin below.

The value of the lateral concrete pressure is regulated by different standards; in the European countries the most used is the German standard DIN When the cement and water chemically react, the concrete starts gaining a certain strength and it can carry part of the weight of the concrete above. The concrete type represents one of the principal factors to take into account for a formwork design.

The crucial role played by the concrete in the formwork design

This pressure is comparable, at least in the first stage, to the pressure that the water exerts against the wall of a basin. It must be noted that, despite formworks are temporary structures, they affect the safety of the workers and the final result. However, for a matter of simplicity, all these additional parameters are neglected by the codes due to their not significate impact on the final load evaluation. To avoid loss of time and money for the job site, formworks require the same level of Engineering required for permanent structures.

Formworks are temporary structures i. As a result of the developments in concrete technology and construction processes in the last years, the times required xin complete a pour are always more and more challenging. As stated previously, the maximum lateral pressure of the concrete, according to DIN Despite the notion of temporary structures, formworks require a detailed and specific design, due to the high loads involved.

To prevent any risk, for the workers and for the final result, formwork should be designed for the ultimate as well for 81218 serviceability limit states.

The classes vary from the stiff F1 to the very flowable F6, up to the self-consolidating concrete SCC. Home About Us Products.

Kern Tunneltechnik – The crucial role played by the concrete in the formwork design

After a certain time indicated with tE in the Standardshowever the concrete starts setting, and consequently the lateral pressure can be neglected. Start-up of the Karlsruhe project. The consistency classes are based on spread values determined per DIN according to the slump-flow test. From this it is easy to understand how the concrete type and quality can impact on the dimensions of the formwork and vin on its costs.